In recent years, for automobile fuel, the ratio of introduction of methyl alcohol, ethyl alcohol, methyl-t-butyl ether, and other alcohols and gasoline in which these alcohols are mixed (flex fuels), so-called “gasohol”, has been increasing as a general trend.
However, these flex fuels have the following defects:
(a) Water is easily contained
(b) Phase separation easily occurs due to an increase in the moisture content and a drop in temperature.
(c) Further, oxidation causes deterioration resulting in production of organic acids and easy phase separation. For example, methanol changes to formic acid and ethanol changes to acetic acid. As a result, phase separation into mainly alcohol and/or organic acid and water occurs, so there is much stronger corrosiveness compared with ordinary gasoline.
In general, as the material for automobile fuel tanks, Pb—Sn alloy plated steel sheet, called “terne-coated sheet”, and zinc (Zn)-plated steel sheet treated with thick chromate has been used. However, Pb—Sn-alloy plated steel sheet and zinc-plated steel sheet are corroded by the moisture which is contained in alcohol, formaldehyde, acetoaldehyde, and other alcohol oxides, formic acid, acetic acid, or other impurities and elute plating ingredients into the fuel. For this reason, conventional plated steel sheet is not suited to a fuel tank for flex fuels.
Further, in recent years, art relating to hot dip aluminum coated steel sheet has been proposed. PLT 1 describes aluminum plated steel sheet for fuel tank use which has excellent corrosion resistance comprised of steel sheet on the surface of which an aluminum plating layer comprised of, by wt %, Mg: 0.5 to 15%, Si: 2 to 15%, and a balance of Al and unavoidable impurities is provided, the aluminum plating layer containing an Mg2Si phase.
PLT 2 describes hot dip aluminum coated steel sheet which has excellent corrosion resistance and appearance characterized by comprising steel sheet on the surface of which an aluminum plating layer comprised of, by wt %, Mg: 1 to 15%, Si: 2 to 15% and also an alkali earth metal element other than Mg in 0.02 to 5% is provided, the plating layer containing an Mg2Si phase.
PLT 3 proposes hot dip aluminum coated steel sheet comprised of steel sheet on the surface of which a metal structure which contains, by wt %, Mg: 3 to 10%, Si: 1 to 15%, and Ca: 0.01 to 2% and which has an Mg2Si phase with a long axis of 10 μm or less is provided.
Even if these plated steel sheets are used for fuel tanks for petroleum-derived gasoline or diesel oil, the inside surfaces of the tanks will not be corroded, so pitting will not occur. Further, no floating corrosion products are formed, so no filter clogging occurs in the fuel circulation system either. Furthermore, the outside surface of the tank is protected against salt corrosion by paint in addition to the plating layer, so again no pitting occurs.
However, if using flex fuels for automobile′ fuel, the fuel tanks will be corroded. That is, with aluminum plated steel sheet, the alcoholate reaction (see following formula) causes the aluminum plating to dissolve in the alcohol, so sufficient corrosion resistance is not exhibited against alcohol or mixed gasoline. Al—Mg—Si-based plated steel sheet is resistant to an alcoholate reaction, but if the alcohol concentration is high, again a sufficient corrosion resistance is not obtained:2Al+6(R—OH)→2(Al(OR)3)+3H2 2(Al(OR)3)+6H2→2Al(OH)3+6(R—H)
As art for solving these problems relating to alcohol and flex fuels, several arts have been proposed. PLT 4 describes steel sheet for a high corrosion resistance fuel tank for alcohol or flex fuels characterized by comprising steel sheet on the surface of which is provided an organic bonded solid lubricant film on which aluminum or an Al—Si alloy plating which contains Si: 3 to 12% is deposited to 30 to 400 g/m2, which is treated by chromate to 10 to 40 mg/m2 converted to chrome, and which contains metal powder over that.
PLT 5 describes steel sheet for fuel tank use comprised of steel sheet on the surface of which a zinc-based plating layer is provided, over which an organic film which is formed by a bisphenol type epoxy resin, phosphoric acid-modified epoxy resin, modified epoxy resin which has a primary hydroxyl group, a glycoluril resin and a metal compound is provided, the metal compound containing at least a V compound, the glycoluril resin being contained in 5 to 20 mass % with respect to the total solid content of the organic film, and the metal compound being contained in 10 to 20 mass %. However, these steel sheets for fuel tank use are coated on their surfaces after plating (surface treated). Plating along is not enough to improve the corrosion resistance to alcohol or flex fuels.
As explained above, plated steel sheet which has sufficient corrosion resistance with respect to alcohol or mixed gasoline of the same has not yet been commercialized.